X-ray tube

ABSTRACT

An X-ray tube having a glow cathode which is surrounded by a focussing device (Wehnelt cylinder), is particularly characterized by the provision of supports which restrain oscillations and which are located close to the glow cathode.

United States Patent [1 1 Friedel et al.

[ [451 Feb. 19, 1974 X-RAY TUBE Inventors: Rudolf Friedel; Herbert Schnitger,

both of Erlangen, Germany Siemens Aktiengesellschait, Erlangen, Germany Filed: Dec. 7, 1972 Appl. No.: 313,093

Assignee:

Foreign Application Priority Data Dec. 27, 1971 Germany P 21 64 811.3

US. Cl. 313/57, 313/55 Int. Cl. l-l0lj 35/06 Field of Search .f. 313/57, 55

[56] References Cited UNITED STATES PATENTS 2,157,718 5/1939 Mutscheller 313/57 4 Claims, 7 Drawing Figures 1 X-RAY TUBE This invention pertains to an X-ray tube having a glow cathode which is surrounded by a focussing device (Wehnelt cylinder).

in order to provide rays extending from a narrowly limited range which are sufficiently intensive for general uses, such as medical diagnosis and therapy, X-ray tubes are usually operated with a glow cathode combined with a focussing device. This device can be also used in the manner of a lock fence to actuate the intensity of the outgoing electrons, particularly to switch them on and off.

It was found that these cathodes of X-ray tubes have the drawback that the glow coil in case of resonance carries out its own oscillations. They can be caused, for example, by shakings taking place during the movements of the rotary anode. These oscillations produce density variations in the bundle of electron rays which appear upon the anode and there can produce local heatings which can be so strong that the annode material will be melted and evaporated. This can cause high voltage discharges which will destroy the X-ray tube. Furthermore, oscillating changes in the electron distribution are undesirable, particularly in diagnosis, since they make worse the image producing properties of the tube. Meltings which do not yet produce a ruptures also damage the focal point path, resulting in a diminution of intensity of radiated rays.

An object of the present invention is to avoid the above-mentioned detrimental effects or at least to reduce them so that they will not be damaging any more.

Other objects will become apparent in the course of the following specification.

In the accomplishment of the objectives of the present invention it was found desirable to provide supports which restrain oscillations quite close to the glow coil. They are not directly connected to the glow coil and due to their short distance from it they prevent heat outflow and thus prevent an influencing of the distribution of the radiated electrons despite their restraint of oscillations.

Experiments which were actually carried out and which provided a guide to the present invention, have surprisingly disclosed that glow cathodes which were considered to be mechanically stable due to the recrystallization of the tungsten wire, can start such oscillations under specific operating conditions that the above-mentioned undesirable conditions take place. As is known, glow cathodes used in X-ray tubes are shaped as coils and thus have the form of a screw spring which can be quite long. Percussions taking place particularly in rotary anodes (unbalance, bearing friction, play of anode in ball bearings, etc.) can cause resonance oscillations in such a glow coil. As is known, the geometrical measurements and the material constants of the coil itself (own oscillations) are determinative. In case of a resonance actuation. i.e., an actuation in the range of own oscillation frequency, substantial amplitudes can be produced even when stimulated by little energy. On the other hand, in a heat coil its own oscillation is not constant. It is dependent upon the heating current, namely, the heating causing a change in the elasticity of the wire as well as its mechanical tension due to the expansion. On the other hand rotary anodes are accelerated from speed zero to their full speed and then braked again to zero when their use is terminated. This makes it necessary to provide all actuating frequences up to the highest frequency determined by the r.p.m.

It was found that two closely located own oscillation frequencies take place at the coil. One oscillation is transverse to the fixing supports of the coil and one oscillation is perpendicular thereto (transverse to the tube axis), i.e., toward the anode. In the first case the coil can lie for a short time against the focussing cylinder. No direct damage results therefrom, aside from a small heat removal, if the coil and the head are insulated one from the other or if they are interconnected by a high ohmic resistance. The cooling taking place at the moment of contact causes a return of the electron emission at the place of contact, which however does not influence substantially the operation of the tube. On the other hand in case of grid'constructed tubes, namely, tubes wherein the focussing head is pulsewise negatively connected relatively to the heating coil for the switching of the tube current, the engagement of the coil with the focussing head breaks the preliminary negative voltage, so that the tube cannot be operated any more for lattice spraying. in addition overchargings of the anode are then produced which quickly cause a complete destruction of the tube.

The oscillation which takes place perpendicularly to the above described one, namely one which extends in the direction of electronic rays, can also be dangerous since it causes a substantial change in the size and the density distribution of the electronic focal point. Thus it can happen that meltings take place upon the anode due to local overheating of the anode material depending upon the load, the special emission properties of a cathode and the oscillation amplitude, which result in the destruction of the focal point path. This diminishes the life of the tubes,'on the one hand due to the diminution of the high voltage resistance by metal coatings produced by streaming, and on the other hand, by the burning of the high voltage path caused by the melting.

The supporting elements of the present invention can be differently constructed, for example, as a simple pin which ends shortly under the glow coil looking from the side of the ray emission. The distance should not be greater than a few l/lO mm to sufficiently dampen the oscillations. its thickness is preferably made larger than the distances between the windings of the coil. The pin serves as a brake only for oscillations extending perpendicularly to the direction of electronic emission. However, side oscillations are also braked if the support is shaped as a thicker pin which is ground round at its end, so that this hollow is somewhat adapted to the diameter of the coil..The same results are achieved by other fork-shaped supports, for example, by bent wires, such as two drilled wires which are spread Y- shaped at their ends, or by the sheet strip cut dovetail at its end. As materials for the supports can be used all materials which are usually used for the making of cathodes. These are particularly heavy melting metals such as tungsten, tantalum, molybdenum, etc. Due to the short contact taking place when the coil swings to the support it is also possible to use electrically insulating materials. The above-mentioned metallic supports can be also coated with insulating substances at the contacting surfaces. However, the insulating substances must have a very high temperature resistance, as for example, zirconium oxide. The number of the supporting elements is so selected, that it suffices to prevent the damaging oscillations depending upon the length of the coils.

The invention will appear more clearly from the following detailed description when taken in connection with the accompanying drawing showing by way of example only, preferred embodiments of the inventive idea.

In the drawing:

FIG. 1 is a partial section through an X-ray tube having a rotary anode and provided with a cathode constructed in accordance with the present invention.

FIG. 2 to 6 are cross-sections through a cathode having different supporting members.

FIG. 1 shows an X-ray tube 1 having a cylindrical vacuum glass container 2 which carries at one end the anode combination 3 and at the other end the cathode combination 4. The cathode combination 4 carries the glow coil 5 in the Wehnelt cylinder 6 and is carried by a support 7 upon the inwardly extending end 8 of the container. Conduits 9 supply current to the coil 5 which consists of tungsten wire which is 0.3 mm thick and is wound into a coil of 2 mm diameter. The anode includes a plate 10 which is rotatably supported by a rotor 11 connected to a support 12 at the other end of the container. The glow coil 5 is fixed in the cathode section 4 in insulators l3 and 14 consisting of A1 0 at the front part 7 of the cathode support. An insulator 16 consisting of Al O is also provided in the part 7' for the pin preventing the oscillations. The pin 15 is located between the insulators 13 and 14, (see also FIGS. 2 and 2a). When the coil 5 begins to oscillate, it is braked by the pin 15 consisting of M0, since it strikes it. When in addition the pin is split at its upper end, as illustrated, and the remaining parts 150 are bent away from each other, a support restraining the oscillations from the side can be also provided. The manufacture is particularly easy since the splitting and the grinding of the side points 15b can take place mechanically.

The pin 15 serving as the support can be also shaped, as shown in FIG. 3, from two wires of tungsten, 0.3 mm thick which are twisted jointly. At the free end the twisting of the wires 17 and 18 is opened and a fork is formed. The two prongs of the fork prevent additionally the side swinging of the coil 5.

On the other hand instead of using wires, the construction of FIG. 4 is a sheet blade 19 made of tungsten sheet 0.5 mm thick the end of which is cut dovetail like and thus prevents the swinging of the coil 5 in the same manner as the wire fork of FIG. 3.

A similar construction is shown in FIG. 5 illustrating a pin 20 of molybdenum which has a thickness OF 2 mm. This pin also prevents an oscillation of the coil 5. Its end located adjacent the coil, is provided with a recess corresponding to the shape of the coil 5.

FIG. 6 shows a twisted coil similar to that illustrated in FIG. 3. However, in the construction shown in FIG. 6 beads 21, 22 of zirconium oxide are provided upon the ends of the two wires which have a diameter of 0.5

mm and are electrically insulating. In this construction it is not necessary to provide an insulation shown in FIGS. 1 and 2 for electrically separating the support from the Wehnelt cylinder since the beads 21, 22 provide an insulating separation.

Correspondingly insulated ends can be also provided in the constructions shown in FIGS. 1, 2, 4 and 5. This is particularly important when the Wehnelt cylinder is used as the lock grid. Furthermore it is simpler for the manufacture to weld or to solder the supports directly with the Wehnelt cylinder. In that case it is not necessary to provide insulating parts corresponding to the part 16 in the part 7' of the cathode construction.

What is claimed is:

1. An X-ray tube having a glow cathode with a glow coil, a focussing device enclosing said glow cathode and supports located closed to said glow coil to restrain oscillations thereof.

2. An X-ray tube in accordance with claim 1, wherein said supports have ends directed to said glow coil which are fork shaped.

3. An X-ray tube in accordance with claim I, wherein said supports have electrically insulating portions interrupting a current flow between said glow coil and said supports.

4. An X-ray tube in accordance with claim 3, wherein said electrically insulating portions are carried by ends of said supports directed to said glow coil. 

1. An X-ray tube having a glow cathode with a glow coil, a focussing device enclosing said glow cathode and supports located close to said glow coil to restrain oscillations thereof.
 2. An X-ray tube in accordance with claim 1, wherein said supports have ends directed to said glow coil which are fork shaped.
 3. An X-ray tube in accordance with claim 1, wherein said supports have electrically insulating portions interrupting a current flow betweeN said glow coil and said supports.
 4. An X-ray tube in accordance with claim 3, wherein said electrically insulating portions are carried by ends of said supports directed to said glow coil. 